This invention relates to a novel composition useful for stripping photoresist polymers from substrates. Furthermore, this invention relates to a method of stripping photoresist polymers with the novel composition.
Printed circuit boards are manufactured by plating a thin layer of copper on an epoxy-glass cloth laminate board of uniform thickness. A predetermined pattern of holes is then drilled to accommodate various electrical components. A film of partially polymerized photoresist plastic is deposited on the laminate over the copper plating. Photoresist films are commonly composed of acrylic resins, polyvinyl cinnamates, diazo compounds, phenol-formaldehydes, or other similar film-forming materials. This film is further polymerized, or cross-linked, by the action of ultraviolet light, into a hard chemically resistant film.
By masking with an appropriate glass or plastic material the resist film is selectively hardened by the exposure to light in specific predetermined areas, while the resist film in the masked areas is left unchanged. The unchanged resist film is then dissolved in a "developer" by a solvent such as 1,1,1-trichloroethane or a solution of butoxyethanol and sodium carbonate or similar solutions. The copper in the cleaned areas may then be removed by etching or additional copper and other metals may be plated thereon. In either event it then becomes necessary to remove the exposed hardened resist film from the laminate.
It is known that such resist films can be removed by the action of strong organic solvents, such as methylene chloride or trichloroethylene. A method of removing resist from printed circuit boards is described in U.S. Pat. No. 3,789,007 wherein the board is treated with a mixture of 85 to 97 percent by weight of methylene chloride with the balance being methanol. Other methylene chloride-containing photoresist stripper compositions are disclosed in U.S. Pat. No. 3,625,763 and 3,813,309. Archer et al., U.S. Pat. No. 4,438,192, disclose an improved photoresist stripper composition which contains methylene chloride, methanol and methyl formate.
Zuber et al., U.S. Pat. No. 4,202,703, disclose that a solution of tetramethylammonium hydroxide and a surfactant in a lower alcohol solubilizes photoresist films without attacking materials found in integrated circuit devices so that a subsequent 1,1,1-trichloroethane rinse completely removes the photoresist.
Briney, U.S. Pat. No. 3,796,602, discloses that photohardened resist masks are stripped from substrate surfaces, particularly metal surfaces of printed circuit boards, using an aqueous stripping liquid heated to near its phase-transition temperature, the composition containing from 1 to 50 percent of a partially miscible organic solvent. Aqueous stripping compositions useful are comprised of water as a major ingredient and at least one organic solvent which is partially miscible in water. Organic solvents found particularly useful include 2-butoxyethanol, ethylene glycol mono-isobutylether, ethylene glycol mono-n-hexylether, 2-propoxy-propanol, 2-(2-butoxyethoxy)ethanol, diethylene glycol mono-n-hexylether, ethylene diacetate, diethyl malonate and furfural. Up to about 0.5 percent by weight of surfactants may be used. It is further taught that it may be desirable to add up to 10 percent of an alkaline agent.
In the many heretofore known stripping compositions for stripping photoresist, there are components which are toxic, corrosive, or are hazardous to use. Furthermore, many of these stripping compositions do not strip the photoresist in an expedient manner.
What is needed is a composition useful in stripping photoresist polymers from substrates which does not contain toxic reagents, does not contain corrosive agents, and does not contain reagents which are hazardous to use. Furthermore, what is needed is a composition which results in faster stripping of the photoresist polymer mask.